The present invention refers to an apparatus for the surface treatment of workpieces by means of a plasma.
In the case of a surface treatment, for example plasma nitration, plasma carburization as well as the coating of workpieces by means of the PVD or CVD method, the surface of the workpiece has to meet high requirements. If the surface of the workpiece is contaminated, oxidized or passivated, a cleaning process has to take place prior to the treatment by a plasma.
If the workpieces are subjected to a machining process prior to the treatment by a plasma, they are intermediately stored subsequent to this process or at least have to be transported from the processing device to the surface treatment device. In this case the workpieces are subject to ambient influences. For example, the treated surfaces oxidize because of the humidity of the air. Therefore, it is indispensable to clean the workpieces prior to surface treatment.
In a cleaning process, impurities such as oils and greases have to be removed. The processes involved are very cumbersome, as the surface of the workpiece might be passivated, especially, if non-volatile impurities are involved. A nitration of aluminum workpieces, for example, is very difficult, as an aluminum oxide layer, acting as a diffusion inhibitor, immediately forms when the aluminum comes into contact with air. As the oxide layer is an adherent layer, it is extremely difficult to remove.
From U.S. Pat. No. 5,216,223, a plasma treatment apparatus is known comprising a treatment chamber which can be evacuated. To insert and remove workpieces into and from the treatment chamber during the operation of the vacuum, the treatment chamber has a sluice arranged upstream thereof for introducing the workpieces and a sluice arranged downstream thereof for removing workpieces. The sluice for removing the workpieces from the treatment chamber comprises two lateral doors leading towards aftertreatment chambers. These aftertreatment chambers each comprise a door leading to the outside. The aftertreatment chambers arranged downstream of the sluice for removing the workpieces serve to aftertreat chemically or thermally the surfaces previously treated in a surface treatment process by means of a plasma. Even in the apparatus known from U.S. Pat. No. 5,216,223, it is necessary to clean the workpieces prior to the introduction thereof into the treatment chamber for a surface treatment by means of a plasma.